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  <title>BRL­CAD Glossary</title>
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  <h1>BRL­CAD Glossary</h1>
  <hr>
  <dt>
    <a class="c1" name="ACYCLIC DIRECTED GRAPH">acyclic directed
    graph</a>
  </dt>
  <dd>
    In an <i>acyclic directed graph</i>, usually called a
    <i>DAG</i>, each node may have arcs pointing to multiple
    <a href="#OBJECT">objects</a>, and each <a href=
    "#OBJECT">object</a> may be pointed to by multiple <a href=
    "#OBJECT">objects</a>. However, no node is permitted to refer
    to any of its ancestors.
  </dd>
  <hr>
  <dt class="c2">air code</dt>
  <dd>
    See <a class="c3" href="#SPACE">space code</a>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="AZIMUTH" id="AZIMUTH">azimuth</a>
  </dt>
  <dd>
    Viewing aspects in <a class="c3" href="#MGED">mged</a> may be
    specified in terms of <i>azimuth</i>, <a href=
    "#ELEVATION">elevation</a>, and <a href="#TWIST">twist</a>.
    <div class="c4">
      <a href="az_el.gif"><img src="az_el_sm.gif"></a>
    </div>
    <p><i>Azimuth</i> is the angle (in degrees) around the
    <i>z</i>-axis through which the viewing direction is rotated
    from the positive <i>xz</i>-quarterplane towards the positive
    <i>y</i>-axis. When <i>azimuth = <a href=
    "#ELEVATION">elevation</a></i> = 0, for instance, viewing is
    parallel to the <i>x</i>-axis in the direction of decreasing
    <i>x</i>. When <i>azimuth</i> = 90 degrees, viewing is parallel
    to the <i>yz</i>-plane along some direction of decreasing
    <i>y</i>: the precise direction within the <i>yz</i>-plane
    being specified by the <a href="#ELEVATION">elevation</a>.
    <i>Azimuth</i> can take on any real value.</p>
  </dd>
  <hr>
  <dt>
    <a class="c1" name="BOOLEAN" id="BOOLEAN">Boolean</a>
  </dt>
  <dd>
    The <i>Boolean</i> (i.e., set-theoretic) operations of
    <i>union</i>, <i>intersection</i>, and <i>subtraction</i> are
    used in <a href="#CSG">constructive solid geometry</a> to build
    complex objects from simpler ones. <img src="bool.gif">
  </dd>
  <hr>
  <dt>
    <a class="c1" name="BREP" id="BREP">boundary representation</a>
  </dt>
  <dd>
    <i>Boundary representation</i> (<i>BREP</i>) is one of the two
    major types of <a href="#MODEL">solid modeling</a>, the other
    being <a href="#CSG">constructive solid geometry</a>.
    <i>BREP</i> objects are built by describing the entire
    enclosing surface of an object. This will typically involve a
    variety of surface types including, for example, planar,
    cylindrical, NURB, and spherical surfaces. While objects in a
    <i>BREP</i> system may be built using <a href=
    "#BOOLEAN">Boolean</a> operations, the resulting object is
    typically stored as a set of enclosing surfaces.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="COMBINATION" id=
    "COMBINATION">combination</a>
  </dt>
  <dd>
    A <i>combination</i> is a formula for combining <a href=
    "#SOLID">primitive solids</a> and/or other <a href=
    "#OBJECT">objects</a> into a more complex shape. Each <a href=
    "#SOLID">solid</a> or other <i>combination</i> directly in this
    formula is referred to as a <a href="#MEMBER">member</a> of
    this <i>combination</i>. The formula may involve
    <i>intersection</i>, <i>subtraction</i>, and <i>union</i>
    operations, and the <i>combination</i> may be assigned a
    <a href="#SHADER">shader</a> and a color. <i>Combinations</i>
    are typically built in <a class="c3" href="#MGED">mged</a>
    using the <a href="mged_cmd_index.html#c">c</a>, <a href=
    "mged_cmd_index.html#comb">comb</a>, <a href=
    "mged_cmd_index.html#r">r</a>, or <a href="mged_cmd_index.html#g">g</a>
    commands.<a href="#REGION">Regions</a> and <a href=
    "#GROUP">groups</a> are special types of <i>combinations</i>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="CSG" id="CSG">constructive solid
    geometry</a>
  </dt>
  <dd>
    <i>Constructive solid geometry</i> (<i>CSG</i>), sometimes
    referred to as <i>combinatorial solid geometry</i>, is one of
    the two major types of <a href="#MODEL">solid modeling</a> (The
    other being <a href="#BREP">boundary representation</a>). In a
    <i>CSG</i> system, complex objects are built by combining
    simpler objects, with primitive solids as the basic building
    blocks. The combinations make use of <a href=
    "#BOOLEAN">Boolean</a> operations, and the resulting objects
    are stored as <a href="#TREE">trees</a>. <i>BRL­CAD</i> is
    actually a hybrid of <i>CSG</i> and <a href="#BREP">BREP</a>,
    but its main emphasis is on the <i>CSG</i> capabilities.
  </dd>
  <hr>
  <dt class="c2">DAG</dt>
  <dd>
    See <a class="c3" href="#DAG">acyclic directed graph</a>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="ELEVATION" id="ELEVATION">elevation</a>
  </dt>
  <dd>
    Viewing aspects in <a class="c3" href="#MGED">mged</a> may be
    specified in terms of <a href="#AZIMUTH">azimuth</a>,
    <i>elevation</i>, and <a href="#TWIST">twist</a>.
    <i>Elevation</i> is the angle (in degrees) through which the
    viewing direction is rotated out of the <i>xy</i>-plane towards
    the positive <i>z</i>-axis (see <a href="#AZIMUTH">the
    illustration in the <i>azimuth</i> entry</a>). When
    <i>elevation</i> = 90 degrees, for instance, viewing is
    parallel to the <i>z</i>-axis in the direction of decreasing
    <i>z</i>. <i>Elevation</i> can take on any real value between
    -90 and +90 degrees.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="GROUP" id="GROUP">group</a>
  </dt>
  <dd>
    A <i>group</i> is a <a href="#COMBINATION">combination</a> with
    all <i>union</i> operators. Typically a <i>group</i> is used to
    combine components into a subsystem, subsystems into a system,
    or systems into top-level collections.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="IDENT" id="IDENT">ident</a>
  </dt>
  <dd>
    An <i>ident</i> (also referred to as <i>component ID</i>,
    <i>region ID</i>, <i>item</i>, or <i>ID code</i>) is an integer
    assigned to a <a href="#REGION">region</a> or <a href=
    "#REGION">regions</a> to encode information used by application
    programs that process <i>BRL­CAD</i> models. <i>Idents</i> are
    assigned by the user and are not directly used by
    <i>BRL­CAD</i>, but are reported to applications that need
    them.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="INHERITANCE" id=
    "INHERITANCE">inheritance</a>
  </dt>
  <dd>
    <i>Inheritance</i> is an attribute of <a href=
    "#COMBINATION">combinations</a> that determines how <a href=
    "#SHADER">shader</a> attributes are handled when a <a href=
    "#MEMBER">member</a> of a <a href=
    "#COMBINATION">combination</a> is itself a <a href=
    "#COMBINATION">combination</a>. If the <i>inheritance</i> flag
    is set to 1, then all the <a href="#SHADER">shader</a>
    attributes of <a href="#COMBINATION">combinations</a> lower in
    the <a href="#TREE">tree</a> are overridden by this <a href=
    "#COMBINATION">combination</a>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="KEY SOLID">key solid</a>
  </dt>
  <dd>
    The <i>key solid</i> is the solid used to provide a reference
    during <a href="#MATRIX%20EDIT">matrix editing</a> operation.
    Rotations, translations, and scaling in this mode are performed
    about a point on the ``key solid''.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="LOS" id="LOS">LOS</a>
  </dt>
  <dd>
    An <i>LOS</i> is a percentage (0 to 100) assigned to a <a href=
    "#REGION">region</a> representing the percentage of the
    <a href="#REGION">region</a> that consists of material
    substance. It might be thought of as the <a href=
    "#REGION">region</a>'s <i>solidity</i>, and is somewhat
    analogous to the alpha channel of a computer graphics display.
    <i>LOS</i> is assigned by the user and is not directly used by
    <i>BRL­CAD</i>, but is reported to applications that need it.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="GIFTMAT" id="GIFTMAT">material code</a>
  </dt>
  <dd>
    A <a href="#REGION">region</a> may be assigned an integer
    <i>material code</i> by the user. This is intended to identify
    the material used to build the part. <i>Material codes</i> are
    assigned by the user and are not directly used by
    <i>BRL­CAD</i>, but are reported to applications that need
    them.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="MATRIX EDIT">matrix edit mode</a>
  </dt>
  <dd>
    This is the mode that <a class="c3" href="#MGED">mged</a> is in
    while editing a transformation matrix in a <a href=
    "#COMBINATION">combination</a>. It is characterized by the
    illumination of one object in the graphics window, and the
    appearance of matrix editing selections in the <i>Edit</i>
    menu. This mode may be entered through the <i>Matrix
    Selection..</i> entry of the <i>Edit</i> menu, or via the
    <a class="c3" href="mged_cmd_index.html#oed">oed</a> command.
    Editing operations such as rotation, translation, and scale may
    be performed in this mode and will affect all objects below the
    edited matrix in the <a href="#TREE">tree</a>. These operations
    will be applied in reference to a <a href="#KEY%20SOLID">key
    solid</a> that is a leaf of that tree.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="MEMBER" id="MEMBER">member</a>
  </dt>
  <dd>
    An <a href="#OBJECT">object</a> is referred to as a
    <i>member</i> of a <a href="#COMBINATION">combination</a> if it
    is directly referenced by the <a href=
    "#COMBINATION">combination</a>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="MGED" id="MGED">MGED</a>
  </dt>
  <dd><i>Mged</i> is the interactive graphical editor for
  <i>BRL­CAD</i>. It may be used to view, modify, and create
  models. Normally, the user views the model in a wireframe display
  in <i>mged</i>.</dd>
  <hr>
  <dt>
    <a class="c1" name="MODEL_COORDINATES" id=
    "MODEL_COORDINATES">model coordinates</a>
  </dt>
  <dd>
    In a <i>BRL­CAD</i> model, the <i>model coordinates</i> are the
    actual coordinates of points on the model in <i>real</i> units.
    The user may position a model at any desired location in
    <i>model coordinates</i>. By convention in <i>BRL­CAD</i>, a
    model is oriented so that the positive <i>x</i>-axis points
    from the model's back to its front, the positive <i>y</i>-axis
    points from the model's right to its left, and, since the
    coordinate system is right handed, the positive <i>z</i>-axis
    points from the model's bottom to its top. All <i>BRL­CAD</i>
    models are stored in millimeters, however the user may operate
    in nearly any units by using the <a class="c3" href=
    "#MGED">mged</a> <a href="mged_cmd_index.html#units">units</a>
    command. See also <a href="#VIEW">view coordinates</a>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="OBJECT" id="OBJECT">object</a>
  </dt>
  <dd>
    <i>Objects</i> are the fundamental entities constituting a
    <i>BRL­CAD</i> model. They are the nodes of the <a href=
    "#DAG">DAG</a> into which the database is organized. The
    terminal nodes, or leaves, of the <a href="#DAG">DAG</a> are
    the <a href="#SOLID">solids</a> and the internal nodes are the
    <a href="#COMBINATION">combinations</a>, of which two types are
    <a href="#REGION">regions</a> and <a href="#GROUP">groups</a>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="PATH" id="PATH">path</a>
  </dt>
  <dd>
    A <i>path</i> to an <a href="#OBJECT">object</a> in
    <i>BRL­CAD</i> is a list of <a href=
    "#COMBINATION">combinations</a> that ends at the desired
    <a href="#OBJECT">object</a>. <i>Paths</i> are specified with a
    syntax similar to the one used in UNIX file systems (with ``/''
    characters as separators). Each element of the <i>path</i> must
    be a <a href="#MEMBER">member</a> of the previous element. For
    example, ``left_arm/arm/hand/thumb'' might be a <i>path</i>
    from ``left_arm'' to ``thumb'' in some <i>BRL­CAD</i> database
    indicating that ``arm'' is a <a href="#MEMBER">member</a> of
    ``left_arm'', ``hand'' is a <a href="#MEMBER">member</a> of
    ``arm'', and ``thumb'' is a <a href="#MEMBER">member</a> of
    ``hand''. The <i>path</i> information implies a set of
    transformation matrices (each <a href="#MEMBER">member</a> of a
    <a href="#COMBINATION">combination</a> has a transformation
    matrix associated with it). As a <i>path</i> is traversed, the
    effects of encountered transformation matrices are accumulated,
    and typically applied to the <a href="#OBJECT">object</a> at
    the end of the <i>path</i>. This means that the ``thumb''
    referred to by the <i>path</i> ``left_arm/arm/hand/thumb'' is
    likely to be in a different location and orientation (and
    possibly scaled) from the ``thumb'' referred to by
    ``right_arm/arm/hand/thumb''.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="SOLID" id="SOLID">primitive solid</a>
  </dt>
  <dd>
    <i>Primitive solids</i> are the basic building blocks of
    <a href="#CSG">CSG</a> <a href="#MODEL">solid modeling</a>
    systems. The <i>primitive solids</i> available in
    <i>BRL­CAD</i> include:
    <ul>
      <li>
        <a href="brlcad_solids.html#ARB">ARB</a>
      </li>
      <li>
        <a href="brlcad_solids.html#ARBN">ARBN</a>
      </li>
      <li>
        <a href="brlcad_solids.html#ARS">arbitrary faceted
        solid</a>
      </li>
      <li>
        <a href="brlcad_solids.html#EBM">extruded bitmap</a>
      </li>
      <li>
        <a href="brlcad_solids.html#RHC">Right Hyperbolic
        Cylinder</a>
      </li>
      <li>
        <a href="brlcad_solids.html#EHY">Elliptical Hyperboloid</a>
      </li>
      <li>
        <a href="brlcad_solids.html#EPA">Elliptical Paraboloid</a>
      </li>
      <li>
        <a href="brlcad_solids.html#RPC">Right Parabolic
        Cylinder</a>
      </li>
      <li>
        <a href="brlcad_solids.html#ETO">Elliptical Torus</a>
      </li>
      <li>
        <a href="brlcad_solids.html#ELL">Ellipsoid</a>
      </li>
      <li>
        <a href="brlcad_solids.html#HAF">Half Space</a>
      </li>
      <li>
        <a href="brlcad_solids.html#HF">Height Field</a>
      </li>
      <li>
        <a href="brlcad_solids.html#NMG">Non­Manifold Geometry</a>
      </li>
      <li>
        <a href="brlcad_solids.html#PART">Particle</a>
      </li>
      <li>
        <a href="brlcad_solids.html#PG">Polysolid</a>
      </li>
      <li>
        <a href=
        "brlcad_solids.html#PIPE%20%3Cli%3E%3Ca%20href=">Spline</a>
      </li>
      <li>
        <a href="brlcad_solids.html#TGC">Truncated General Cone</a>
      </li>
      <li>
        <a href="brlcad_solids.html#TOR">Torus</a>
      </li>
      <li>
        <a href="brlcad_solids.html#VOL">VOL</a>
      </li>
    </ul>
    <div class="c4"><img src="prims.gif"></div>
  </dd>
  <hr>
  <dt>
    <a class="c1" name="SOLID EDIT">solid edit mode</a>
  </dt>
  <dd>
    This is the mode that <a class="c3" href="#MGED">mged</a> is in
    while editing an individual solid. It is characterized by the
    illumination of one solid in the graphics window, and the
    appearance of solid editing selections in the <i>Edit</i> menu.
    This mode may be entered through the <i>Solid Selection..</i>
    entry of the <i>Edit</i> menu, or via the <a class="c3" href=
    "mged_cmd_index.html#sed">sed</a> command. Solid specific editing
    may be performed in this mode, as well as rotations,
    translations, and scaling.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="QUATERNION" id="QUATERNION">quaternion</a>
  </dt>
  <dd>
    Internal to <a class="c3" href="#MGED">mged</a>, the viewing
    orientation is handled in terms of <i>quaternions</i>. A
    <i>quaternion</i> is a method for representing an arbitrary
    rotation without the ambiguity at the poles that accompanies
    some methods. A <i>quaternion</i> may be thought of as a vector
    (defining an axis of rotation), and a rotation about that axis.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="RAYTRACE" id="RAYTRACE">ray trace</a>
  </dt>
  <dd>
    <i>Ray tracing</i> is the main method for interrogating
    <i>BRL­CAD</i> models. It is performed by selecting a start
    point and a ray direction, then calculating the intersections
    of the resulting ray with all the <a href="#SOLID">primitive
    solids</a> it encounters. The segments of the ray that
    intersect each <a href="#SOLID">primitive solid</a> are then
    combined according to the <a href="#BOOLEAN">Boolean</a>
    expressions for each <a href="#REGION">region</a> that refers
    to those <a href="#SOLID">solids</a>. The result is a series of
    intervals on the ray, called <i>partitions</i>, that intersect
    each <a href="#REGION">region</a> along the ray. If the
    objective of the <i>ray tracing</i> effort is to produce an
    image, the calculations will typically be stopped after the
    first <a href="#REGION">region</a> is encountered. The
    <i>BRL­CAD</i> <a href="#RT">rt</a> program performs such
    <i>ray tracing</i>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="REGION" id="REGION">region</a>
  </dt>
  <dd>
    A <i>region</i> is a <a href="#COMBINATION">combination</a>
    with special significance for application programs that process
    <i>BRL­CAD</i> models. It corresponds to a physical entity of
    uniform material properties. In addition to all the attributes
    of any <a href="#COMBINATION">combination</a>, a <i>region</i>
    also has an <a href="#IDENT">ident</a> number, a <a href=
    "#MATCODE">material code</a> number, an <a href="#LOS">LOS</a>
    number, and a <a href="#SPACE">space code</a> number. No two
    <i>regions</i> in a model should share any points in space.
    Such <i>overlaps</i> are reported, for instance, by the
    <i>BRL­CAD</i> <a href="#RAYTRACE">ray tracer</a>, <a class=
    "c3" href="#RT">rt</a>.
  </dd>
  <hr>
  <dt class="c2">region ID</dt>
  <dd>
    See <a class="c3" href="#IDENT">ident</a>.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="RGB" id="RGB">RGB</a>
  </dt>
  <dd><i>RGB</i> is an abbreviation for <i>Red</i>, <i>Green</i>,
  and <i>Blue</i>. When a color is specified in <i>BRL­CAD</i>, it
  is in terms of its primary color components. Each component may
  be from 0 to 255. An <i>RGB</i> of ``0 0 0'' is black, ``255 255
  255'' is white, and ``0 255 0'' is green.</dd>
  <hr>
  <dt>
    <a class="c1" name="RT" id="RT">rt</a>
  </dt>
  <dd>
    <i>Rt</i> is one of the programs in the <i>BRL­CAD</i> package.
    It is used to <a href="#RAYTRACE">ray trace</a> a model and
    produce a color shaded image. <i>rt</i> supports transparency,
    reflection, refraction, shadows, texture mapping, light
    sources, and animation.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="SHADER" id="SHADER">shader</a>
  </dt>
  <dd>
    A <i>shader</i> is a <i>BRL­CAD</i> module that performs
    calculations to determine the actual pixel color to be
    displayed when a ray intersects an <a href="#OBJECT">object</a>
    during rendering by <a href="#RT">rt</a>. <i>Shaders</i> are
    assigned to <a href="#COMBINATION">combinations</a> by the
    user, and some allow additional parameters to adjust their
    appearance. Available <i>shaders</i> include <i>mirror</i>,
    <i>glass</i>, <i>plastic</i>, and <i>texture</i> (for texture
    mapping). Source code modules in the <i>rt</i> directory with
    names of the form <i>sh_*.c</i> correspond to the available
    shaders.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="MODEL" id="MODEL">solid model</a>
  </dt>
  <dd>A <i>solid model</i> is a description of an object or objects
  produced in such a way that any point can be classified without
  ambiguity as lying inside, outside, or on the surface of a
  modeled object. This means that a true solid model cannot have
  surfaces that do not completely enclose space, and solid objects
  that are defined in the model must not be defined based on
  irrelevant variables such as viewing direction.</dd>
  <hr>
  <dt>
    <a class="c1" name="SPACE" id="SPACE">space code</a>
  </dt>
  <dd>
    A <i>space code</i> (or <i>air code</i>) is assigned to
    <a href="#REGION">regions</a> that are intended to represent
    ``empty air'' rather than solid parts of the model. The code
    number may be used to distinguish different air spaces from one
    another. Regions representing solid parts must have a space
    code of 0. <i>Space codes</i> are assigned by the user and are
    not directly used by <i>BRL­CAD</i>, but are reported to
    applications that need them.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="TREE" id="TREE">tree</a>
  </dt>
  <dd>
    A tree is a <a href="#BOOLEAN">Boolean</a> expression
    representing some aggregation of <a href="#OBJECT">objects</a>.
    Strictly speaking, this is an abuse of the term <i>tree</i>:
    such <a href="#BOOLEAN">Boolean</a> expressions are not trees
    in either the computer-science or the graph-theoretic sense,
    but are more correctly referred to as <a href="#DAG">acyclic
    directed graphs</a>, also known as <a href="#DAG">DAG</a>s.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="TWIST" id="TWIST">twist</a>
  </dt>
  <dd>
    Viewing aspects in <a class="c3" href="#MGED">mged</a> may be
    specified in terms of <a href="#AZIMUTH">azimuth</a>, <a href=
    "#ELEVATION">elevation</a>, and <i>twist</i>. <i>Twist</i> is
    the angle (in degrees) around the screen-<i>z</i>-axis through
    which the (model-) <i>z</i>-axis is rotated from the positive
    screen-<i>y</i>-axis towards the positive screen-<i>x</i>-axis.
    (see <a href="#AZIMUTH">the illustration in the <i>azimuth</i>
    entry</a>). When <i>twist</i> = 0, for instance, the positive
    <i>z</i>-axis is some nonnegative scalar multiple of the
    positive screen-<i>y</i>-axis.
  </dd>
  <hr>
  <dt>
    <a class="c1" name="VIEW_COORDINATES" id=
    "VIEW_COORDINATES">view coordinates</a>
  </dt>
  <dd>
    When an <a href="#OBJECT">object</a> is displayed in the
    <a class="c3" href="#MGED">mged</a> graphics window, it is
    converted from <a href="#MODEL_COORDINATES">model</a> to
    <i>view</i> coordinates. The <i>view coordinate</i> system has
    its origin at the center of the viewing cube (the center of the
    <a class="c3" href="#MGED">mged</a> display). The <i>view</i>
    <i>x</i>-axis always points to the right, the <i>view</i>
    <i>y</i>-axis always points toward the top of the display, and
    the <i>view</i> <i>z</i>-axis always points out of the screen
    towards the viewer. The visible portion of view space (known as
    the ``viewing cube'') extends from -1.0 to +1.0 along each of
    the <i>view</i> axes.
  </dd>
  <hr>
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